Timing generators are used to synchronize the many functions found within electronic systems. Most commonly, a certain number of operations must be performed in a given interval of time, this interval of time, generally a cycle time, is marked by a start and a stop pulse or a cycle pulse whose duration is equal to the time between start and stop pulses. It is the purpose of the timing generator to divide the cycle time into sub-intervals, usually of equal length, so that the number of sub-intervals equals the number of operations to be performed. This process is performed in synchronism with the start/stop or cycle pulses. An example of sub-interval timing is in the division of a spinning motor's shaft rotation into 360 equal intervals, one for each degree of rotation. The fact that the rotational speed of the motor's shaft can change, requires that the timing generator adapt its interval timing rate to the varying shaft speed. In this sense, the timing generator is considered to be adaptive and, as such, must vary the time between sub-intervals to accommodate the change in cycle, or shaft rotation time.
The conventional approach to adaptive timing generation for these types of devices has been the phase locked loop (PLL). The dynamic range of the PLL, that is, its ability to accommodate large changes in cycle time, is relatively constrained by noise and saturation in the feedback loop, and in the limited range of the PLL's phase comparator. In electrophotographic copiers and material scanning systems, this necessitates manual adjustment of controls for document or material width, and often does not permit the most efficient use of image memory.